1. Field of the Invention
This invention is directed generally to screws for orthopedic use in humans, and specifically to screws made of cortical bone and method of manufacture thereof.
2. Description of the Related Art
The mammalian skeleton includes dense structural bone known as cortical bone. In humans, the femur, one of the larger long bones of the body, may have a cortical thickness as great as 6–8 mm. Larger mammals, which have been used to provide bone for surgical use in humans, have bones with cortical thicknesses substantially greater than those found in humans.
Screws for orthopedic use may be used for a multitude of purposes, including to join separated bone portions, and to attach various orthopedic implants, such as skeletal plates, to bones. Such screws have commonly been made of surgical quality metals (e.g. stainless steel, surgical grade titanium, and titanium alloys), ceramics and various plastics including some that are bioresorbable.
Metal screws typically remain in the body unless explanted by a later, separate operative procedure, and can inter alia potentially irritate tissue proximate the metal screws, shed ions harmful to the body, back-out or loosen causing injury within the body. Metal screws also can interfere with optimal visualization of the affected area by various diagnostic modalities such as x-rays, CAT scans or MRIs. On the other hand, resorbable screws made of bioresorbable plastic materials which can be absorbed by the body over time have had both limited applicability and success. Resorbable plastic screws have often been limited by insufficient strength, an inability to be formed into a sharp thread-form, an unpredictable absorption rate, an inability to maintain sufficient structural integrity for an adequate period of time, the elicitation of an undesirable inflammatory response, and the potentially toxic effects of the degradation products of the material released by the bioresorption process.
There is therefore a need for a bone screw that combines the advantages of being sufficiently strong so as to be useful for skeletal fixation and for the attachment of various implants to the human skeleton, that additionally is resorbable within the body, and which does not have all the undesirable qualities encountered with screws of the past.